Cetothere Skeletons From the Miocene Choptank Formation 



of Maryland and Virginia 



1. THE SKELETON OF A MIOCENE CHOPTANK CETOTHERE 



Discoveiy of additional mysticete skulls in Oligocene 

 liiarine formations would furnish a much more authori- 

 tative basis for sjjeculative opinions relative to their 

 early geological history. The apically attenuated tri- 

 angular supraoccipital shield of Cetotheriopsis linti- 

 anus (Brandt, 1873, pi. 19), which was recovered near 

 Linz, Austria, from the white quartz sands of the 

 Oligocene Chattian stage, represents an advanced phase 

 in the remodeling of the mysticete skull. The pro- 

 nounced forward thrust of the ix>st«rior cranial bones of 

 this skull clearly supfwrts the conclusion that the re- 

 modeling of the cranial architecture of these Tertiaiy 

 whalebone whales had progi^ssed beyond the initial 

 oven'iding of the intertemix>ral region by the supraoc- 

 cipital prior to the Miocene. 



Furthermore, three distinct types of posterior cranial 

 remodeling represented by Morenocetus parvtis (Ca- 

 brera, 1926, fig. 1 ) , '■'■Plesiocetus''' dyticus ( Cabrera, 1926, 

 fig. 3), and Aglaocetus m-oreni (Kellogg, 1934), and for 

 which no prior geological antecedents are as yet known, 

 have been excavated in the lower Miocene Patagonian 

 marine fomiation. 



The skull of Aetiocetus cotylalveus (Emlong, 1966), 

 which was discovered in the upj^er Oligocene marine 

 Yaquina formation of northwestern Oregon, and which 

 has the horizontally divided jxisterior end of the max- 

 illary projecting backward above and below the su- 

 praorbital process of the frontal, a typical mysticete 

 interlocking structural relationship, cannot be consid- 

 ered to be in the direct antecedent lineage of the mysti- 

 cetes, but does suggest what may have been basic 

 ancestral skull architecture. This Oligocene cetacean 

 seems to have had the usual diphyodont mammalian 

 dentition, the crowns of the cheek teeth having serrated 

 edges. Vestigial cheek teeth are present in the jaws of 

 fetal Recent mysticetes. On all archaeocete skulls thus 

 far described, including Protoeetus, Prozeuglodon, 



Hafiilosauru.H. Donidon, and Zygorhiza, molar teeth 

 are located on the hinder end of the maxillary, which 

 projects backward beneath the supraorbital process of 

 the frontal, and not anterior to this process as in 

 Patrhcetus, M icrozeuglodon, and Aetiocetus^ and prob- 

 ably also in Agorophius and Archueodelphis. 



The direction, elongation, and curvature of the upper 

 and lower transverse processes of the sixth cervical 

 vertebra indicate that the cervical extension of the 

 thoracic retia mirabilia had been previously developed 

 and acquired by this Oligocene Aetiocetus. 



On the AetiocetuH skull the transversely narrowed 

 posterior ends of the palatine bones, separated medi- 

 ally by the vertical plate of the vomer, are prolonged 

 backward beyond the level of the anterior end of the 

 pterygoid fossa, much farther backward than on the 

 skull of any known Eocene archiiecK^ete. Tliis backward 

 prolongation of the palatines increased the length of 

 the internal choanae. Tlie narial fossa in front of the 

 elongated nasal bones is situated at the middle of the 

 length of the skull. The conformation of the bones en- 

 closing the internal choanae may not, however, possess 

 any particular phylogenetic significance. Tlie backward 

 shift of the external nares and the associated narial 

 fossa in the mesorostral trough does not ap{>ear to have 

 any causal lelationship with the prolongation of the 

 internal choanae on all skulls of Recent cetacea:is. For 

 instance, the palatines on the skull of the finback, Bala- 

 enoptera physalus, are not extended backward beyond 

 the level of the anterior end of the pterygoid fossa 

 although the relatively small nasal bones behind the 

 narial fossa are situated almost entirely ix>sterior to 

 the level of the anterior end of the orbit. Conversely on 

 the skull of the bowhead, Balaena mysticetus, the pala- 

 tines extend backward to the occipital condyles and the 

 narial fossa is located far forward on the rostrum, 

 approximately behind the middle of the length of the 



